I have found similar questions in this and this articles, but neither can solve my problem.
The message given by Apple Store Connect was also "ITMS-90725: SDK Version Issue - This app was built with the iOS 14.4 SDK. All iOS apps submitted to the App Store must be built with the iOS 15 SDK or later, included in Xcode 13 or later."
However, I have checked that I am using xcode 13.2.1 which works with iOS 15.2 SDK, running on macOS Big Sur 11.6.4 (well.... a 2015 iMac, kind of old though). I believe it is capable of building a project with iOS 15.2 SDK, but I just can't find where I shall tune (if necessary) to avoid it keeping using iOS 14.4 SDK to build my project. Could you please help me solve the issue? Thank you very much!!